The Abyss and Fortress Sieges are a big part of high-level gameplay in Aion. You should be sure and read both articles on the official Game Guide. Here are two important things to keep in mind.
The Abyss Ranking system works like a leader board once your character hits the Officer ranks. Only a limited number of positions on this leader board are available. This means if there are players with more Glory Points than you, and all positions are filled, you would need to wait for another player to lose enough points for you to advance, or gain more points to overtake the player in the lowest position. If either of these have been achieved, and are maintained, the leader board will reflect your position with regard to your rank.
Fortress Siege Medals
Silver, Gold, Platinum, Mithril, and Ceramium Medals are received for successfully attacking or defending fortresses. The type and number of medals your character will receive is based on which fortress is involved and the grade earned during the siege.
Your character’s grade is determined by contributions and actions during the battle. Your siege grade can't be viewed directly. The number of Abyss Points (AP) acquired during the siege can give you a rough indication of how you did, but AP earned doesn't necessarily correspond entirely with siege ranking.